A direct current (DC) or rectified alternating current (AC) is applied to an electromagnet attached by mechanical means to a boom to attract and hold ferrous metals. The magnet is then moved to another location and the current is removed from the magnet coil to release the metal. However, the magnet core is not completely demagnetized when all the current is removed, so some amount of current must be applied for some period of time in a direction opposite to the original current flow to release all the metal. This results in a “clean drop”.
Originally, a mechanical contact arrangement was employed to apply and reverse the magnet current. These contacts were expensive and needed frequent replacement. Solid-state switching devices have been used more recently to drive current in one direction for the “lift” phase, then in the other direction for the “reverse” phase. This requires solid-state devices of high current carrying capacity and capable of withstanding the voltages involved, usually 230 Volts DC. A small magnet may draw 15 Amperes while a large magnet may draw 80 Amps or more.
An industrial lifting magnet has an inductance L and a resistance R. The resistance and inductance are distributed throughout the length of the coil, but the magnet is electrically equivalent to a resistance in series with a purely inductive element. If a magnet having initial current I(0) is shorted out, the current as a function of time is I(t)=I(0)exp(−Rt/L). In other words, the current in a magnet has a characteristic period of L/R. The value of L/R for industrial lifting magnets is typically about 0.5 seconds
A problem occurs when the current in the magnet coil is reduced or cut off. The collapsing magnetic field in the inductor produces a current that must be discharged through an external impedance. The current through this impedance can produce a voltage transient that can be very large, and will damage semiconductor switching devices if not controlled.
It is well known that the voltage transient caused by reducing the current in an inductor can be suppressed by placing a “flyback diode” across the inductor, but such a diode cannot be permanently connected across a lifting magnet because it would short out the power supply when the applied voltage is reversed.
One means of mitigating this problem, if power is supplied by a DC generator, is to reverse the field of the DC generator and thereby reverse the output. But the generator field is also an inductor, so transient suppression is still required, and reversing the field results in a demagnetization time that is unacceptably long.
Most relevant related U.S. Patents:
U.S. Pat. No. 4,306,268—Essentially an H bridge with relays for switching a DC source, but had a forward flyback diode in series with resistors permanently connected across the magnet, which would have conducted heavily during reverse. Another flyback diode for reverse, again in series with a resistor, was switched in using relays. Voltage drop on these resistors represented the decaying current in the magnet, and when the current was low enough, reverse voltage from the DC source began flowing into the magnet via a diode that kept the source isolated from the magnet up to that point. But voltages reached up to 1000 V, and there must have been severe arcing in the relay contacts.
U.S. Pat. No. 4,600,964—A design using two magnet coils, one for lift and one for drop. This used full-wave rectified output from an AC generator, which was switched between lift and drop coils using relays. The main problem with this invention is that magnets are expensive, and most magnets already in the field have only one coil. Flyback diodes on each coil are necessary to prevent arcing when the relay contacts are opened. Furthermore, flyback diodes alone without a secondary discharge would discharge the magnet too slowly.
U.S. Pat. No. 5,325,260—This design used an AC generator that was connected to a standard bridge rectifier via mechanical relay contacts. The rectified AC from the bridge was applied to the magnet using mechanical relays in a standard H bridge configuration. Before the lift or drop relay contacts were opened, the relay contacts feeding the standard rectifier bridge were opened, thus causing the H bridge rectifiers to act like flyback diodes. This reduced the stored magnetic energy before the lift or drop contacts opened, and would reduce the high voltage arcing to some extent. But there was no secondary discharge circuit as in the present invention, and no capacitor across the magnet, which means either there was a large high voltage transient or a relatively long time was required to reverse the magnet current. Special mercury-wetted relay contacts were required because of the contact arcing.
U.S. Pat. No. 7,495,879—A solid state design that used insulated gate bipolar transistors (IGBTs) in an H bridge configuration and a DC power source. The stored magnetic energy at the end of a lift or drop was fed into a large capacitor, then the capacitor was discharged through a fifth IGBT and resistor. To avoid excessive high voltage, the capacitor must have been very large and must have had a rather high voltage rating. The stored magnetic energy was dissipated in a resistor, not the magnet, so the resistor must have been of high wattage rating, and therefore of large physical size and must have required a large heat sink.
U.S. Pat. No. 7,697,253—Another solid-state control using a DC generator and an H bridge configuration. This design allegedly dissipated the stored magnetic energy in the DC generator and a resistor in series with a transient voltage supressor (TVS), not in the magnet resistance. This would produce some extra wear on the generator, and would have required a very large TVS to withstand twice the lifting current for at least several tenths of a second.